Dense wavelength division multiplexing (DWDM) requires optical spectrum analyzers (OSAs) that have higher spectral resolution than is typically available with current OSAs. For example, grating-based OSAs and autocorrelation-based OSAs encounter mechanical constraints, such as constraints on beam size and the scanning of optical path lengths, which limit the degree of resolution that can be obtained.
As an alternative to grating-based and autocorrelation-based OSAs, optical heterodyne detection systems can be utilized to monitor DWDM systems. FIG. 1 is a depiction of a prior art optical heterodyne detection system. The optical heterodyne detection system includes an input signal 102, an input waveguide 104, a local oscillator signal 106, a local oscillator waveguide 108, an optical coupler 110, an output waveguide 118, a photodetector 112, and a signal processor 116. The principles of operation of optical heterodyne detection systems are well known in the field of optical heterodyne detection and involve monitoring the heterodyne term that is generated when an input signal is combined with a local oscillator signal. The heterodyne term coexists with other direct detection signals, such as intensity noise from the input signal and intensity noise from the local oscillator signal.
Optical heterodyne detection systems are not limited by the mechanical constraints that limit the grating based and autocorrelation based OSAs. The spectral resolution of an optical heterodyne system is limited by the linewidth of the local oscillator signal, which can be several orders of magnitude narrower than the resolution of other OSAs.
In order to improve the performance of optical heterodyne detection systems with regard to parameters such as sensitivity and dynamic range, it is best for the heterodyne signal to have a high signal to noise ratio. However, the signal to noise ratio of the heterodyne signal is often degraded by noise that is contributed by the direct detection signals, especially in the case where the input signal includes multiple carrier wavelengths. One technique for improving the signal to noise ratio of the heterodyne signal, as described in U.S. Pat. No. 4,856,899, involves amplifying the input signal before the input signal is combined with the local oscillator signal in order to increase the amplitude of the heterodyne signal. Although amplifying the input signal increases the amplitude of the heterodyne signal, the amplification also increases the intensity noise of the input signal and may not improve the signal to noise ratio of the heterodyne signal.
It is also important in optical heterodyne detection that the polarization of the input signal and the local oscillator signal are matched. In order to match the polarization of the local oscillator signal to the polarization of the input signal, the polarization state of the local oscillator signal may be controlled by a polarization controller 120 as indicated by the two loops in the heterodyne detection system of FIG. 1. A disadvantage of the optical heterodyne detection system of FIG. 1 is that detection of the input signal is highly dependent on the polarization of the input signal.
A polarization diversity receiver can be incorporated into an optical heterodyne detection system to provide polarization independent signal detection. Although a polarization diversity receiver provides polarization independent signal detection, the polarization diversity receiver does not provide a way to separate the intensity noise from the heterodyne signal. In order to improve the performance of heterodyne detection systems, it is necessary to be able to clearly distinguish the heterodyne signal from the intensity noise.
In view of the prior art limitations in optical heterodyne detection systems, what is needed is an optical heterodyne detection system that generates a heterodyne signal with an improved signal to noise ratio.